Limited angle, brushless DC rotary actuators are well known and commercially available. One such actuator is disclosed in U.S. Pat. No. 5,512,871 issued Apr. 30, 1996. Another is disclosed in U.S. Pat. No. 6,313,553 issued Nov. 6, 2001.
The actuators now available and as presented by the above-identified references, are known commercially as “torque motors”, and include a ferromagnetic yoke attached to a magnet comprising the rotating member. The yoke is deemed necessary in order to strengthen the rotating member sufficiently for operation, and to close the attached magnet's magnetic circuit. But the presence of a yoke results in a structure with a high moment of inertia, reducing dynamic performance, and having a single-sided stator structure which creates unbalanced magnetic forces on the rotating member attracting it toward the stator.
One drawback of such an actuator is that approximately 20% or more of the power consumption is used to move its own inertia, reducing dynamic performance. Depending on the application duty cycle, this wasteful power consumption value can be even higher.
A second drawback is that due to the unbalanced design, the rotor magnet has a magnetic attraction force toward the stator structure, which leads to the use of an expensive thrust ball bearing to perform an axial stop function yet allowing the requisite rotational freedom of the rotor. A part of this axial attraction force is useful to withstand vibration when used in certain applications, but in any case the rotor rotation is subjected to friction torque such that some of the magnetic force used to create rotation and output torque is wasted.
A third drawback is the electromagnetic flux leakage. It can be clearly understood by seeing the structure of the prior art as in FIG. 1 and FIG. 2, in that the 4 coils and stator poles are necessarily located very close to each other. Even with a small magnetic airgap between the rotor yoke and poles, when saturation appears at high currents necessary for the creation of high torque, most of the coil flux does not pass through the magnet, which would create the torque, but instead closes itself on the neighboring coil.
A fourth drawback is that in servo applications, the actuator magnet cannot be used to activate the position sensor receiver, but a second, separate magnet is required to be attached to the yoke, adding cost and weight to the rotating member.
A fifth drawback is that due to the physical space required for the construction and assembly of the stators and coils, the actual useful stroke of the prior art's 4-pole single-sided actuator is approximately 75 degrees compared to a theoretical stroke of 90 degrees, which would be preferable in most applications. Use of the prior art's 2-pole construction in the same physical actuator size could produce a 90 degree usable stroke, however the torque would be reduced by 50% rendering such an actuator undesirable and unusable in many applications. And, increasing the size of the actuator to attain the required torque would also render the actuator undesirable due to the resulting size, weight and cost.